This could also happen if a thumbnail is missing or is not saved in the correct BMP format.
Check to see if all of your thumbnails are there. If not, that will be the issue. The thumbnails appear to move long to the next available asset. It's strange, but, it happens.
If they are all there, then I would advise checking they are all in the correct format.
I noticed that when I saved the thumbnails from GIMP, they wouldn't show in the editor, even though they were the correct size and were bitmap images. All I did to remedy this was, right click on the thumbnail, click edit (so it opens in MS Paint), then just click save. Job done. It would appear that any bitmaps saved in MS Paint at the correct BMP format.
I hope this helps.
Cheers
Jay.